Along with its long legs and flowing mane, most of us would list the sound of clip-clopping hooves as one of a horse’s most distinctive features. Only one group of animals have evolved this single toe, and those sturdy hooves have enabled horses to out-run predators and made them attractive prospects for domestication. We take a closer look at the structure of the hoof and why humans decided horses needed shoes.
Over millions of years, all four-limbed, land-based vertebrates (including humans) evolved from a common ancestor with five toes. Following different selection pressures in varied landscapes, the foot took on many forms. However, only one living group of animals – Equus – ended up with a single toe per foot. Equus contains seven living species of horses, donkeys, and zebras (collectively termed equids). Historically, as the body mass of equids increased, their centre toes got bigger and more resistant to stress, while side toes shrunk. Eventually this led to a hoof. Having a single hoof at the end of each long leg enables equids to run fast on open plains and evade predators.
Hoof anatomy
A horse’s hoof has an outside, underside, and inside. On the outside is the hoof wall – a hard outer covering made of keratin (like fingernails and rhino horn) that is continually growing and protects the structures within the foot. As it grows, the hoof wall is naturally worn down by abrasive surfaces. Beneath the outer wall is the inner wall, which is softer and expands with movement to provide shock absorption.
On the underside of the hoof are the sole, frog, and the bars. The most noticeable structure is the frog, which is tough and triangular-shaped pointing towards the horse’s toe. The frog provides shock absorption, aids traction, and protects the digital cushion on the inside of the hoof. This digital cushion is made up of cartilaginous material, providing further shock absorption. On the inside of the hoof there are also a series of bones that contribute to its shape and stability.
Natural hoof wear
Wild and free-ranging horses walk and graze continuously over vast distances and a diverse range of terrains. With such continual stimulation the frog becomes thicker and other hoof structures become stronger – much like our feet after a summer of walking barefoot. Regular movement, and contraction and expansion of the hoof, increases circulation and lymphatic drainage to and from the area. There is also significant natural shock absorption that helps protect against concussion-related tendon and joint damage in barefoot or wild horses. However, horse’s hooves can lose their effectiveness when the animals are kept on soft, wet ground, and work on a single terrain. Horses can also suffer from bruised soles if traveling far or over stony ground.
Horseshoe revolution
As horses were domesticated, people developed horseshoes to prevent excessive damage to the hooves of their working animals. Horseshoes date back to around 400BC and were originally made of rawhide or plants. Metal horseshoes were designed in Europe and became commonplace by the 13th and 14th centuries. They give horses extra traction on muddy surfaces and to protect their hooves from softening in wet ground.
Barefoot or shoes
Shoeing of horses has maintained a stronghold in modern horse-keeping even where horses are predominantly kept for sport or leisure. In many cases, shoeing is a tradition perpetuated by modern horse management despite increased stabling and reduced workload on unvaried terrain. However, there is a movement towards barefoot horse-keeping, which can work well for horses with structurally sound hooves, and with appropriate horse management.
Sophie May Watts is a science writer based in the UK.
